There’s a phenomenon in the field of health care called the "obesity paradox," which says that obese or overweight people with certain medical conditions, such as diabetes, kidney disease, and high blood pressure, are protected from death. New research by Samuel Preston, a professor in the Department of Sociology in the School of Arts & Sciences and a member of the University’s Population Studies Center, challenges this finding.

“Obesity is not protective against death for patients with cardiovascular disease,” says Preston, who collaborated with Boston University’s Andrew Stokes. The pair looked at more than 30,400 participants of the National Health and Nutrition Examination Survey between 1988 and 2011, more than 3,300 of whom had cardiovascular disease.

Previous research findings claim that for those with cardiovascular disease, being obese not only isn’t a bad thing, it could actually be beneficial. Preston says the theory is that a person with cardiovascular disease and fat stores will perhaps outlive someone who does not.

“It’s conceivable that there are health advantages,” he says. “But we show in this paper that they are overwhelmed by the disadvantages of being obese once you control for two sources of bias.”

The two sources of bias are weight history and smoking.

Most studies of this nature only take into account weight at the time of the study—even if someone has lost a significant amount of body mass. For their work, published online in the journal Obesity, Preston and Stokes opted to include weight history.

“Almost every study that’s been published is just based on a single snapshot of weight,” Stokes says. “There’s no attempt to introduce weight histories into these analyses, but it turns out to have a profound effect.”

Specifically, it allowed the researchers to distinguish between people who were never obese from those who were formerly overweight/obese and lost weight.

“People who have lost weight, who have moved from the category of overweight/obese to normal weight, have exceptionally high mortality,” Preston says.

To go one step further, the scientists excluded smokers, only incorporating lifelong non-smokers into the research. Combined, these two factors led to a dramatic reversal of the obesity paradox.

This isn’t the first time Preston and Stokes have found similar results. Their current work builds on a previous study on diabetes published in 2014 in the journal Epidemiology. Cardiovascular disease, however, is what they describe as a bigger disease process, and it’s more often fatal and the leading cause of death in the United States.

The researchers say their work can have consequences for clinical care because physicians may be taking into account the obesity paradox when making decisions about how to treat patients.

“Most physicians are aware of this paradox,” Preston says. “There is just a lot of confusion about why this is occurring.”